The present invention relates to radiological phantoms, and in particular, to a radiological phantom for use in X-ray mammography.
Radiological mammographic examinations provide very early detection of the formation of cancerous deposits in the tissue of the breast. In order to perform such examinations, X-rays are passed through the breast tissue and expose an X-ray film. When developed, the film will contain an image of the tissue mass of the breast. Two mammographic indications of breast cancer, microcalcifications and soft tissue fibrillar extensions, are denser and more radiopaque than normal breast tissue and therefore, if present, will show up as light areas on the developed film. When detected early, these growths will be extremely small, as will the X-ray images they produce. Consequently, it is frequently difficult to discern the images produced by these growths from the image produced by the surrounding tissue. In order that the images of these small growths be reproduced clearly so as to be readily identifiable on the exposed film, it is essential that the radiologist know the overall performance and imaging capability of the radiological system so that it may be adjusted for peak performance.
An evaluation of the system is typically accomplished by exposing a stable test device, commonly referred to as a phantom, to the X-ray beam and examining the resultant image produced on the film. Generally, phantoms are designed to simulate the radiographic characteristics of a particular tissue mass. Thus, in the field of mammography, phantoms are particularly designed to provide a substantially equivalent attenuation of an X-ray beam as would breast tissue.
Mammographic phantoms typically include a resolution section and frequently include a step wedge section as well. The resolution section gauges the ability of the X-ray system to detect extremely small objects. Thus, the resolution section contains test objects of known size, frequently consisting of a plurality of small fibers and particles of varying sizes which are more opaque to X-rays than the surrounding phantom structure. These fibers and particles are designed to simulate microcalcifications and soft tissue fibrillar extensions which provide mammographic indications of breast cancer. Upon exposure of the phantom to an X-ray beam, the fibers and particles will produce images on the film; some fibers and particles will produce clearer images than others, while the smallest fibers and particles will not produce visible images at all. By determining the size of the smallest particle or fiber whose image is clearly produced on the film, the radiologist will know the limit of his system for detecting these warning signs of breast cancer.
The step wedge section consists of a series of regions having progressively increasing radiolucency which are used to determine the quality of the X-ray beam by gauging image contrast. Several mammography phantoms incorporate step wedges consisting of an array of holes, normally five (5), of varying depth. By comparing the optical densities of the circular images produced by the holes on both the reference film and test film, the beam quality can be evaluated. An example of a mammographic phantom having both the aforementioned resolution section and step wedge features is the Model No. 76-001 Mammographic QA Phantom produced by Nuclear Associates of Carle Place, N.Y.
Radiological systems for mammography basically consist of a flat horizontal support surface which defines the imaging plane of the system. As used herein, the term "imaging plane" refers to the region directly above the support surface where those objects whose images are to be reproduced on an X-ray film are positioned. Suspended above the support surface is an X-ray source from which X-ray beams are directed downwardly, through the object and support surface, to expose a film plate located in a holder beneath the support surface. During a mammographic examination, a breast would be positioned in the imagining plane so that the chest wall is adjacent one edge of the support surface. Thus, as the X-ray beams pass through the breast they create an image thereof on the film below. Similarly, when utilizing a phantom to evaluate the overall performance and imaging capability of the radiological system, the phantom is placed in the imagining plane so that upon exposure to the X-ray beam an image of the phantom will be created in the film.
With currently available mammography phantoms, such as the aforementioned phantom produced by Nuclear Associates, the radiologist must first examine the film in order to determine the orientation of the image before being able to identify which of the test objects were clearly reproduced.
Additionally, it is difficult to consistently locate currently available phantoms so that the step wedge is always in the same position relative to the X-ray beam. It is desirable to locate the phantom so that the step wedge is adjacent the center of the X-ray beam where the effects of beam scattering are minimized. As the position of the step wedge changes, the effect of beam scattering on the contrast measurements produced by the step wedge will vary, and hence the measurements will be less reliable.
Consequently, a need exists for a solution to these problems.